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| United States Patent |
5,625,095
|
|
Miura
, et al.
|
April 29, 1997
|
Process for producing high purity acetic acid
Abstract
A high purity acetic acid is prepared by reacting methanol with carbon
monoxide in the presence of a rhodium catalyst, iodide salts, and methyl
iodide, wherein an acetaldehyde concentration in the reaction liquid is
maintained at 400 ppm or lower. This may be attained by contacting the
liquid containing carbonyl impurities with water to separate and remove
the carbonyl impurities. After that, the liquid can be returned to the
reactor.
| Inventors:
|
Miura; Hiroyuki (Hyogo, JP);
Shimizu; Masahiko (Hyogo, JP);
Sato; Takashi (Hiroshima, JP);
Morimoto; Yoshiaki (Niigata, JP);
Kagotani; Masahiro (Hyogo, JP)
|
| Assignee:
|
Daicel Chemical Industries, Ltd. (Osaka, JP)
|
| Appl. No.:
|
458348 |
| Filed:
|
June 2, 1995 |
Foreign Application Priority Data
| Jun 15, 1994[JP] | 6-132724 |
| Jun 20, 1994[JP] | 6-137213 |
| Jun 30, 1994[JP] | 6-149652 |
| Jul 06, 1994[JP] | 6-154401 |
| Aug 22, 1994[JP] | 6-196524 |
| Current U.S. Class: |
562/519; 562/608 |
| Intern'l Class: |
C07C 051/12 |
| Field of Search: |
562/519,608
|
References Cited
U.S. Patent Documents
| 4008131 | Feb., 1977 | Price.
| |
| 4615806 | Oct., 1986 | Hilton.
| |
| 5001259 | Mar., 1991 | Smith et al.
| |
| 5155265 | Oct., 1992 | Scates et al.
| |
| 5214203 | May., 1993 | Koyama et al.
| |
| 5371286 | Dec., 1994 | Blay et al. | 562/519.
|
| Foreign Patent Documents |
| 0265140 | Apr., 1988 | EP.
| |
| 487284 | Nov., 1991 | EP.
| |
| 0497521 | Aug., 1992 | EP.
| |
| 0638538 | Feb., 1995 | EP.
| |
| 1063133 | Mar., 1967 | GB.
| |
Primary Examiner: Geist; Gary
Assistant Examiner: Williams; Rosalynd
Attorney, Agent or Firm: Flynn, Thiel, Boutell & Tanis, P.C.
Claims
What we claim is:
1. A process for producing a high purity acetic acid, comprising the steps
of continuously reacting methanol with carbon monoxide in the presence of
a rhodium catalyst, an iodide salt, and methyl iodide, wherein the
reaction is carried out while maintaining an acetaldehyde concentration in
the reaction liquid at 400 ppm or lower.
2. The process as described in claim 1, wherein the reaction is carried out
by removing acetaldehyde from a process liquid being circulated into a
reactor to maintain the acetaldehyde concentration in the reaction liquid
at 400 ppm or lower.
Description
FIELD OF THE INVENTION
The present invention relates to a novel process for producing high purity
acetic acid formed by carbonylating methanol in the presence of a rhodium
catalyst. More specifically, the present invention relates to a novel
process for producing high purity acetic acid, wherein organic iodides and
carbonyl impurities contained in acetic acid formed by rhodium-catalyzed
carbonylation are reduced.
DESCRIPTION OF THE RELATED ART
Various processes are known as an industrial process for producing acetic
acid. Among them, a process for producing acetic acid by continuously
reacting methanol with carbon monoxide in the presence of water using a
rhodium catalyst and methyl iodide is the best process.
Recently, reaction conditions and catalyst improvement have been
investigated, and processes for industrially producing acetic acid at a
high productivity are disclosed, wherein catalyst stabilizers such as
iodide salts are added and the reaction is carried out at a lower water
content than conventional (U.S. Pat. No. 5214203 and U.S. Pat. No.
5001259). It is disclosed therein that the water content in a reaction
liquid is reduced to decrease by-products such as carbon dioxide and
propionic acid. However, there is the problem that other trace impurities
increase in amount as the productivity of acetic acid increases and
deteriorate the quality of product acetic acid. In particular, in a
quality test by which the amounts of very minute reducing impurities
present in acetic acid are checked, which is called a permanganate
reducing substance test (permanganate time), minute increase in impurities
having minute concentrations, which are hard to quantitatively determine,
even with high-grade instrumental analysis, can be detected, and these
impurities lead to deterioration of product quality.
Impurities which particularly exert influences on different kinds of
applications are contained as well in these trace impurities. For example,
it is known that in a process for producing vinyl acetate from ethylene
and acetic acid, the impurities deteriorate a palladium series catalyst
used in the process. These impurities include carbonyl compounds and
organic iodides. To be specific, it is known that they include carbonyl
compounds such as acetaldehyde, butylaldehyde, crotonaldehyde, and
2-ethylcrotonaldehyde, aldol condensation products thereof, and alkyl
iodides such as ethyl iodide, butyl iodide, and hexyl iodide (EP-A
487284).
However, these carbonyl impurities which deteriorate permanganate time have
boiling points tightly close to those of iodide catalyst accelerators, and
it is difficult to sufficiently remove alkyl iodides which deactivate
catalysts for producing vinyl acetate by ordinary means such as, for
example, distillation.
In view of the forgoing, there are disclosed conventional techniques such
as the treatment of crude acetic acid containing these minute reducing
impurities with ozone and oxidizing agents. However, treatment with ozone
and oxidizing agents have limits in the concentrations of the impurities
to be treated. For example, compounds generated by decomposing unsaturated
compounds such as crotonaldehyde and 2-ethylcrotoaldehyde by ozone
processing are saturated aldehydes. Aldehydes themselves have reducing
properties and are nothing but compounds which deteriorate the
permanganate time. Accordingly, refining such as distillation and
treatment with active carbon is required after treatment with ozone in
order to remove saturated aldehydes (U.S. Pat. No. 5155265).
It is known as well to treat crude acetic acid with macro reticulated
strong acid cation exchange resins, or strongly acidic cation exchange
resins, substituted with silver to remove organic iodides (U.S. Pat. No.
4615806). While this method is effective for removing alkyl iodides,
hydrogen iodide, and inorganic iodide salts, it is insufficient for
removing the unsaturated carbonyl impurities described above.
While in every method described above, crude acetic acid is processed, it
is attempted as well to remove carbonyl impurities contained in a process
circulating liquid in a continuous reaction process. That is, a method for
removing carbonyl impurities is disclosed, wherein a methyl iodide
recirculating stream to a carbonylation reactor is reacted with amino
compounds which react with carbonyl impurities to form water soluble
nitrogen-containing derivatives, and an organic methyl iodide phase is
then separated from an aqueous derivative phase, followed by distilling
the methyl iodide phase to remove carbonyl impurities (EP-A 487284).
However, the concentration of the carbonyl impurities contained in an
organic stream recirculated into the carbonylation reactor described above
is still high, and therefore it is not clear if the carbonyl impurities
have been sufficiently removed. Further, a new problem of removing
nitrogen-containing compounds is involved.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a flow diagram of a reaction used for the carbonylation of
methanol to acetic acid-acetic acid recovery system.
FIG. 2 shows one example of a distillation system for separating methyl
iodide from acetaldehyde.
In the drawings,
10: Carbonylation reactor.
12: Flasher.
14: Methyl iodide--acetic acid splitter column.
30: Lower phase in liquid separator.
40, 60: Distillation columns.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for producing
high purity acetic acid, wherein carbonyl compounds or organic iodides
which are impurities of acetic acid as described above are reduced by
controlling conditions of a reactor in which they are generated.
Further, an object of the present invention is to provide an effectual,
specific means to carry out such control.
The present inventors have noted that a great part of the impurities
described above originate in acetaldehyde generated during the reaction
and that these impurities are formed in the reactor. That results a
finding that both carbonyl compounds and organic iodides, which are
impurities contained in the resulting acetic acid, can be reduced by
controlling the acetaldehyde concentration in a reactor to thereby obtain
high purity acetic acid, thus completing the present invention.
The invention provides a process for producing a high purity acetic acid,
comprising the steps of continuously reacting methanol with carbon
monoxide in the presence of a rhodium catalyst, an iodide salt, and methyl
iodide, wherein the reaction is carried out while maintaining an
acetaldehyde's concentration in the reaction liquid at 400 ppm or lower.
It is preferable that the above defined reaction is carried out by removing
acetaldehyde from the process liquid being circulated into a reactor to
maintain the acetaldehyde concentration in the reaction liquid at 400 ppm
or lower.
The invention moreover provides a process for producing a high purity
acetic acid comprising the steps of reacting methanol with carbon monoxide
in the presence of a rhodium catalyst, an iodide salt and methyl iodide,
separating the resulting reaction liquid into a volatile phase containing
acetic acid, methyl acetate and methyl iodide and a low volatile phase
containing the rhodium catalyst, distilling the volatile phase to obtain a
product mixture containing acetic acid and the overhead containing methyl
acetate and methyl iodide, and recirculating said overhead into the
reactor, wherein the overhead or a condensate of the carbonyl impurities
of said overhead is contacted with water to separate it into an organic
phase containing methyl acetate and methyl iodide and an aqueous phase
containing the carbonyl impurities containing acetaldehyde, and said
organic phase is recirculated into the reactor.
The invention, in addition, provides a process for producing a high purity
acetic acid comprising the steps of reacting methanol with carbon monoxide
in the presence of a rhodium catalyst, an iodide salt and methyl iodide,
separating the resulting reaction liquid into a volatile phase containing
acetic acid, methyl acetate and methyl iodide and a low volatile phase
containing the rhodium catalyst, distilling the volatile phase to obtain a
product mixture containing acetic acid and the overhead containing methyl
acetate and methyl iodide, and recirculating said overhead into the
reactor, wherein the overhead containing acetaldehyde and methyl iodide is
distilled at a top temperature of 55.degree. C. or higher, at a reflux
tank's temperature of 25.degree. C. or higher, at a pressure of 1
kg/cm.sup.2 or more, and acetaldehyde is separated and removed to be
recirculated into the reactor.
The invention besides provides a process for producing high purity acetic
acid comprising the steps of reacting methanol with carbon monoxide in the
presence of a rhodium catalyst, an iodide salt and methyl iodide,
separating the resulting reaction liquid into a volatile phase containing
acetic acid, methyl acetate and methyl iodide and a low volatile phase
containing the rhodium catalyst, distilling the volatile phase to obtain a
product mixture containing acetic acid and an overhead containing methyl
acetate and methyl iodide. and recirculating said overhead into the
reactor, wherein the overhead containing acetaldehyde and methyl iodide is
distilled at a top temperature of less than 55.degree. C. and a reflux
tank's temperature of less than 25.degree. C. in the presence of an
alcohol and acetaldehyde is separated and removed to be recirculated into
the reactor. It is preferable that methanol is introduced at a lower
position than a stage charged with the overhead containing acetaldehyde
and methyl iodide.
A preferable embodiment of the invention is to maintain an acetaldehyde's
concentration in the reaction liquid at 400 ppm or lower by separating the
resulting reaction liquid into a volatile phase containing acetic acid,
methyl acetate and methyl iodide and a low volatile phase containing the
rhodium catalyst, distilling the volatile phase to obtain a product
mixture containing acetic acid and the overhead containing methyl acetate
and methyl iodide, and recirculating said overhead into the reactor,
wherein the overhead or a condensate of the carbonyl impurities of said
overhead is contacted with water to separate it into an organic phase
containing methyl acetate and methyl iodide and an aqueous phase
containing the carbonyl impurities containing acetaldehyde, and said
organic phase is recirculated into the reactor.
Another preferable embodiment of the invention removes acetaldehyde and
methyl iodide by the overhead containing acetaldehyde and methyl iodide
being distilled at a top temperature of 55.degree. C. or higher, at a
reflux tank's temperature of 25.degree. C. or higher, at a pressure of 1
kg/cm.sup.2 or more, and acetaldehyde is separated and removed to be
recirculated into the reactor. Alternatively, the overhead containing
acetaldehyde and methyl iodide is distilled at a top temperature of less
than 55.degree. C. and a reflux tank's temperature of less than 25.degree.
C. in the presence of an alcohol and acetaldehyde is separated and removed
to be recirculated into the reactor.
As shown above, the overhead is distilled under specified conditions to
separate and remove acetaldehyde, and thereafter recirculated into the
reactor.
The term, the low volatile, includes non-volatile.
First, the process for producing acetic acid according to the present
invention will be explained.
The rhodium catalyst used in the present invention is present in a reaction
liquid in the form of a rhodium complex. Accordingly, the rhodium catalyst
may be used in any form as long as it is changed to a complex which
dissolves in the reaction liquid. To be specific, rhodium iodine complexes
and rhodium carbonyl complexes such as RhI.sub.3 and [Rh(CO).sub.2 I.sub.2
].sup.- are effectively used. The amount used is from 200 to 1,000 ppm,
preferably 300 to 600 ppm in terms of concentration in the reaction
liquid.
In the present invention, an iodide salt is added particularly for
stabilizing the rhodium catalyst under low water content and suppressing
side reactions. This iodide salt may be any one as long as it generates an
iodine ion in a reaction liquid. The examples thereof include alkaline
metal iodide salts such as LiI, NaI, KI, RbI, and CsI, alkaline earth
metal iodide salts such as BeI.sub.2, MgI.sub.2, and CaI.sub.2, and
aluminum group metal iodide salts such as BI.sub.3 and AlI.sub.3. Organic
iodide salts can be used besides the metal iodide salts and include, for
example, quaternary phosphonium iodide salts (methyl iodide adducts or
hydrogen iodide adducts of tributyl phosphine and triphenyl phosphine),
and quaternary ammonium iodide salts (methyl iodide adducts or hydrogen
iodide adducts of tertiary amine, pyridines, imidazoles, and imides). In
particular, the alkaline metal iodide salts such as LiI are preferred. The
use amount of the iodide salts is 0.07 to 2.5 mole/liter, preferably 0.25
to 1.5 mole/liter in terms of iodide ion in a reaction liquid.
In the present invention, methyl iodide is used as a catalyst accelerator
and caused to be present in a reaction liquid in an amount of 5 to 20
weight %, preferably 12 to 16 weight %.
A water content in a reaction liquid in the present invention is 15 weight
% or less, preferably 10 weight % or less, and more preferably 1 to 5
weight %.
As the reaction in the present invention is a continuous reaction, methyl
acetate formed by reacting raw material methanol with acetic acid is
present in 0.1 to 30 weight %, preferably 0.5 to 5 weight %, and the
balance of principal components in the reaction liquid is acetic acid
which is a product as well as a reaction solvent.
In the present invention, the typical temperature in the carbonylation of
methanol is about 150.degree. to 250.degree. C., and temperature ranges of
about 180.degree. to 220.degree. C. are preferred. A partial pressure of
carbon monoxide can be changed in a wide range and is typically about 2 to
30 atm, preferably 4 to 15 atm. The whole reactor pressure resides within
a range of about 15 to 40 atm because of the partial pressures of the
by-products and the vapor pressure of liquid contained therein.
The process of the present invention will be explained below based on a
drawing.
FIG. 1 is a flow diagram showing a reaction--acetic acetic acid recovery
system used for rhodium-catalyzed carbonylation of methanol to acetic
acid.
The reaction from methanol to acetic acid--acetic acid recovery system
shown in FIG. 1 includes a carbonylation reactor 10, a flasher 12, and a
methyl iodide--acetic acid splitter column 14. Usually, reaction liquid
contents are automatically maintained at a fixed level in the
carbonylation reactor 10. Fresh methanol and a sufficient amount of water
are continuously introduced into this reactor according to necessity, and
at least a measurable concentration of water is maintained in a reaction
solvent. An alternative distillation system can also be used as long as it
is equipped with means for recovering crude acetic acid, and means for
recirculating a catalyst liquid, methyl iodide, and methyl acetate into
the reactor.
In a preferred process, carbon monoxide is continuously introduced into an
immediate lower part of a stirrer used for stirring contents in the
carbonylation reactor 10. A gaseous supplying material is dispersed all
over the reaction liquid. A gaseous purge stream is discharged from the
reactor to prevent the accumulation of gaseous by-products and maintain a
set partial pressure of carbon monoxide in the whole fixed reactor
pressure. The reactor temperature is automatically controlled, and a
carbon nonoxide-supplying material is introduced at a reaction rate
sufficient for maintaining the preferred reactor pressure. Liquid products
are withdrawn from the carbonylation reactor 10 at a speed sufficient for
maintaining a fixed level and introduced at an intermediate point between
the top and bottom of the flasher 12 via a line 11.
A catalyst liquid is withdrawn from the flasher 12 as a bottom stream 13
(acetic acid containing mainly the rhodium catalyst and iodide salts
together with small amounts of methyl acetate, methyl iodide, and water)
and returned to the carbonylation reactor 10. An overhead 15 from the
flasher 12 contains mainly product acetic acid together with methyl
iodide, methyl acetate, and water.
Product acetic acid (can be withdrawn as a bottom stream) withdrawn as a
side stream close to the bottom of the methyl iodide--acetic acid splitter
column 14 is further refined by methods known by persons having ordinary
skill in the art. An overhead 20 from the methyl iodide--acetic acid
splitter column 14 containing mainly methyl iodide and methyl acetate as
well as small amounts of water and acetic acid is recirculated into the
carbonylation reactor 10 via a line 21. The overhead 20 is typically
separated into two liquid phases by condensing when a sufficient amount of
water is present. A lower phase 30 comprises mainly methyl iodide and
small amounts of methyl acetate and acetic acid, and an upper phase 32
comprises mainly water, acetic acid, and a small amount of methyl acetate.
In the present invention, it is important in the reaction--acetic acid
recovery system to carry out the reaction while keeping an acetaldehyde
concentration in the reaction liquid at 400 ppm or less. An acetaldehyde
concentration exceeding 400 ppm is not preferred because impurity
concentrations in acetic acid, which is a product, increase, and a
complicated refining processing step is required. A method in which
reaction conditions are managed and a method in which acctaldehyde is
removed from a process liquid circulated into a reactor are available in
order to maintain the acetaldehyde concentration in the reaction liquid at
400 ppm or less.
The management of the reaction conditions includes increasing the hydrogen
partial pressure, water concentration, and rhodium catalyst concentration.
These operations mainly lower the acetaldehyde concentration in the
reaction liquid in the carbonylation reactor 10, which results in
controlling an aldol condensation of acetaldehyde and decreasing
by-production speeds of reducing substances such as crotonaldehyde and
2-ethylcrotonaldehyde, and alkyl iodides such as hexyl iodide. However, in
some cases, these methods have a defect of increasing a by-production
speed of propionic acid.
In view of the forgoing, in order to control the acetaldehyde concentration
in the reaction liquid in the carbonylation reactor to 10 to 400 ppm or
less, it is prefertable to remove acetaldehyde from the process liquid
circulated into the carbonylation reactor 10.
The method in which acetaldehyde is removed and the method in which the
reaction conditions are controlled can be used in combination.
Hydrogen partial pressure in the carbonylation reactor 10 originates in
hydrogen generated in the system by aqueous gas shift reaction in the
present reaction and, in some cases, originates in hydrogen introduced
into the reactor together with raw material carbon monoxide.
A method for removing acetaldehyde from the process liquid circulated into
the carbonylation reactor 10 includes methods such as distillation and
extraction, or the combination thereof, and distillation/extraction.
Preferred as the process liquid, which is a target for removing carbonyl
impurities containing acetaldehyde, are the upper phase 32 of the
condensate of the overhead 20, the lower phase 30, which is rich in methyl
iodide, a homogeneous liquid of the overhead 20, if the overhead 20, is
not separated into two layers, an absorbing liquid for vent gas in a waste
gas absorbing system, and a low boiling liquid obtained by further
distilling crude acetic acid liquid withdrawn from the line 17 close to
the bottom of the splitter column 14, because the concentrations of
acetaldehyde are high. Of them, most preferred is the upper phase 32, the
lower phase 30, the homogeneous liquid of the overhead 20, if the overhead
20 is not separated into two layers, or the carbonyl impurities
concentrate thereof. Crude acetic acid liquid withdrawn from the line 17
is usually turned to product acetic acid after the crude acetic acid
liquid is dehydrated in the subsequent distillation column and then
introduced into an acetic acid product column for distilling to separate
high boiling and low boiling matters.
The process liquid which is a target for removing carbonyl impurities
containing acetaldehyde as described above usually contains methyl iodide
of 5 to 90 weight %, acetaldehyde 0.05 to 50 weight %, methyl acetate of 0
to 15 weight %, acetic acid of 0 to 80 weight %, moisture of 0.1 to 40
weight %, and other carbonyl impurities.
The process liquid containing acetaldehyde and others contains useful
components such as methyl iodide, methyl acetate and the like and
therefore is circulated to the carbonylation reactor 10 for reuse.
Accordingly, the acetaldehyde concentration in the reactor can be reduced
by separating and removing acetaldehyde from these circulating liquids.
A method for separating carbonyl impurities containing acetaldehyde
includes a method in which a process liquid containing acetaldehyde is
distilled and separated in one distillation column, a method in which low
boiling components comprising acetaldehyde and methyl iodide are first
separated from other components by distillation, and then acetaldehyde is
Further separated from methyl iodide by distillation, and a method in
which utilizing a property that acetaldehyde is well miscible with water
and methyl iodide is scarcely miscible with water, extraction with water
is employed for separating methyl iodide from acetaldehyde.
When acetaldehyde is separated directly from the process liquid in a single
distillation column, it is pretty difficult to concentrate acetaldehyde
because the boiling point of methyl iodide is close to that of
acetaldehyde. The concentration of acetaldehyde by distillation in a
nonaqueous system such as methyl iodide not only generates paraldehyde and
metaldehyde and prevents acetaldehyde from concentrating but also
deposites metaldehyde in the process and prevents stable operation. In
view of the foregoing, the method in which extraction with water is used
for separating methyl iodide from acetaldehyde is preferred, and
particularly preferred is a method in which, after an acetaldehyde liquid
containing methyl iodide is separated from a process liquid by
distillation, acetaldehyde is selectively extracted with water, and this
is further separated from a distillation/separation process. According to
this method, acetaldehyde can be very efficiently concentrated and removed
because the distillation temperature is high in the concentration of a
water extract by distillation, and an increase in hydrogen ion
concentration in a distillate due to the decomposition of ester can
suppress the generation of paraldehyde and metaldehyde. When distillation
for separation is carried out in one distillation column, water may be
charged into the distillation column, and/or the distillation temperature
and pressure may be elevated to control the generation of paraldehyde and
metaldehyde. Further, distillation conditions may be varied to positively
generate paraldehyde and metaldehyde, and acetaldehyde may be separated
and removed from the bottom products in the forms of paraldehyde and
metaldehyde. In this case, solvents dissolving metaldehyde, such as
methanol, have to be charged into the column to prevent clogging caused by
the crystallization of metaldehyde.
The method in which extraction with water is used For separating methyl
iodide from acetaldehyde will be explained below in detail.
In this water extraction method, carbonyl impurities contained in the lower
phase 30 in a liquid separator containing carbonyl compounds such as, for
example, acetaldehyde, crotonaldehyde, and butylaldehyde are separated
from a reaction product by extracting them with water to form a
recirculating stream containing no carbonyl impurities. According to a
preferred embodiment, the lower phase 30 in the liquid separator bath is
separated into an organic phase recirculating stream containing methyl
iodide and an aqueous phase stream containing carbonyl impurities,
particularly acetaldehyde by extraction with water, and carbonyl
impurities are then removed From the organic phase recirculating stream to
the reactor.
At the first step in the preferred method, the lower phase 30 in the liquid
separator bath containing carbonyl impurities such as, for example,
acetaldehyde, crotonaldehyde and butylaldehyde is contacted to water to
extract the carbonyl impurities into an aqueous phase. The carbonyl
impurities can be determined by an analysis before processing. The
extraction is carried out at temperatures of 0 to 100.degree. C. for 1
second to 1 hour. Any pressure can be employed. Pressure is not essential
in this method, and advantageous conditions can be selected in terms of
cost. There can be used as an extractor, every suitable apparatus which is
known in terms of technique, such as a combination of mixers and settlers,
the combination of static mixers and decanters, RDC (rotated disk
contactor), a Karr column, a spray column, a packed column, a perforated
plate column, a baffle column, and a pulsation column.
After passing through an extractor to a decanter, the aqueous phase stream
containing carbonyl impurities and the organic phase stream containing no
carbonyl impurities are obtained. The organic phase stream is recirculated
to the carbonylation reactor. The aqueous phase stream is sent to a
distillation column to separate the carbonyl impurities from water, and
water is recirculated to the extractor. The value of the carbonyl
impurities removed can be determined by an analysis method.
Next, the distilling method under particular conditions for separating
methyl iodide and acetaldehyde is detailed.
Investigations intensively made by the present inventors have resulted in
finding that the generation and deposition of paraldehyde and metaldehyde
which are the condensation products of acetaldehyde can be controlled, and
methyl iodide can efficiently be separated from acetaldehyde by
controlling top temperature, reflux tank temperature, and pressure or
controlling top temperature and reflux tank temperature in the presence of
alcohol in distilling a mixed liquid containing acetaldehyde and methyl
iodide, and completing the present invention.
That is, the present invention provides a process for efficiently
separating acetaldehyde and methyl iodide by distilling a mixed liquid
containing acetaldehyde and methyl iodide, for example the overhead
described above, at top temperatures of 55.degree. C. or higher, reflux
tank temperatures of 25.degree. C. or higher, and pressures of 1
kg/cm.sup.2 or more, or distilling it at top temperatures of less than
55.degree. C. and circulating tank temperatures of less than 25.degree. C.
in the presence of alcohol.
Followings, FIG. 1 and FIG. 2 are used to illustrate.
A recirculating stream 21 can be formed by the lower phase 30, the upper
phase 32 or, if they are not separated, the whole overhead 20, or
combining these phases and overhead withdrawn from the methyl
iodide--acetic acid splitter column 14 with other recirculated products
containing methyl iodide, methyl acetate, water, and impurities.
The lower phase 30, the upper phase 32 or the whole overhead 20 withdrawn
from the methyl iodide--acetic acid splitter column 14, or the
recirculating stream 21 is introduced into a distillation column 40 and
subjected to the processing of the present invention. Every suitable
equipment which is known in terms of techniques can be used for
distillation columns and separation. The number of stages distillation
columns may be any number. Two or more distillation columns may be used to
carry out the processing of the present invention if it can not be carried
out in a single distillation column for reasons of facilities cost.
The case where the processing of the present invention is carried out in
the two distillation column will be explained below referring to FIG. 2.
The lower phase 30, the upper phase 32 or the whole overhead 20 withdrawn
from the methyl iodide--acetic acid splitter column 14, or the
recirculating stream 21 is introduced into the distillation column 40, and
a methyl iodide recirculating stream withdrawn from the bottom of the
column is recirculated into the reactor via a line 46. A distillate 44 is
obtained from the top.
The distillate 44 from the distillation column 40 is introduced into a
distillation column 60 and subjected to the processing of the present
invention. A methyl iodide recirculating stream from which most of
acetaldehyde has been removed is recirculated into the upper part of the
distillation column 40 via a line 66. Or, in the case where a liquid from
which most of the acetaldehyde has been removed and which is rich in
methyl iodide is obtained from the top, a top distillate is recirculated
into the distillation column 40.
Usually, the process liquid of the whole overhead 20 withdrawn from the
methyl iodide--acetic acid splitter column 14 contains methyl iodide of 5
to 90 weight %, acetaldehyde of 0.05 to 50 weight %, methyl acetate of 0
to 15 weight %, acetic acid of 0 to 80 weight %, moisture of 0.1 to 40
weight %, and other carbonyl impurities.
Because the process liquid containing acetaldehyde described above contains
useful components such as methyl iodide and methyl acetate, it is
circulated into the carbonylation reactor 10 for reuse. Accordingly, after
separating and removing acetaldehyde as much as possible from these
process liquids, they are preferably circulated into the reactor.
If acetaldehyde is not sufficiently removed, acetaldehyde accumulates in
the process liquid, and the aldol condensation of acetaldehyde is
promoted, which result in accelerating the by-production speeds of
reductive substances such as crotonaldehyde and 2-ethylcrotonaldehyde and
alkyl iodides such as hexyl iodide and therefore lead to obtaining product
acetic acid containing these impurities in a large amount.
The separation of acetaldehyde and methyl iodide is difficult because the
boiling points of acetaldehyde and methyl iodide are close to each other,
and in addition, the concentration of methyl iodide by distillation in a
nonaqueous system not only generates paraldehyde and metaldehyde and
prevents acetaldehyde from concentrating but also deposits metaldehyde in
the process and prevents stable operation.
Paraldehyde is a trimer of acetaldehyde and is a liquid having a boiling
point of 124.degree. C. and a melting point of 10.degree. C. In general,
paraldehyde is liable to be generated from acetaldehyde at low
temperatures of 0 to -10.degree. C., and critical generation temperature
is 55.degree. C. It was confirmed in a laboratory that paraldehyde was
generated at 20.degree. C.
Metaldehyde is a tetramer through a hexamer of acetaldehyde and is a white
acicular crystal having melting points of 140.degree. to 246.degree. C.
Metaldehyde is formed at lower temperature than paraldehyde and is
generally generated at a degree of -10.degree. to -40.degree. C. It was
confirmed in a laboratory that metaldehyde was generated at a degree of
5.degree. C. a temperature of -40.degree. C. or less causes
polymerization. Paraldehyde and metaldehyde have stereoisomers, and it was
confirmed that they had different melting point and solubility to
solvents.
As shown here, the generation of paraldehyde and metaldehyde is influenced
by temperature. That is, controlling the operation pressure and operation
temperature in a distillation column has made it possible to separate and
remove acetaldehyde.
That is, it has been found that distilling at top temperatures of 5.degree.
C. or higher, reflux tank temperatures of 25.degree. C. or higher, and a
pressure of 1 kg/cm.sup.2 or more in a distillation column can control the
generation of paraldehyde and metaldehyde and improves the separation
efficiency of methyl iodide from acetaldehyde. Further, shortening the
residence time for returning to the distillation column from a top
condenser through a reflux tank is effective as well for suppressing the
generation of paraldehyde and metaldehyde.
Further, it has been found that since distilling at top temperatures of
less than 55.degree. C. and reflux tank temperatures of less than
25.degree. C. converts acetaldehyde to paraldehyde and metaldehyde at the
top, which have higher boiling points and therefore are moved to the
bottom, acetaldehyde can be removed from bottom products in the form of
paraldehyde and metaldehyde. However, because metaldehyde is a solid
having a low solubility, particularly to methyl iodide, and is deposited,
it clogs not only the perforated plates and packing of the distillation
column but also respective nozzles, pipelines and valves and hinders
operation. The present inventors have found that metaldehyde is dissolved
in alcohols such as methanol, ethanol, and propanol. That is, distillation
in the presence of alcohols has made it possible to prevent clogging.
There may be used any alcohols in the present invention including aliphatic
alcohols such as methanol, ethanol, and propanol, aromatic alcohols such
as benzyl alcohol, and polyhydrlc alcohols such as ethylene glycol.
Methanol which is also used as a raw material is preferred.
Detailed investigations on the solubility of metaldehyde have resulted in
finding that solubilities increase in the order of methyl
iodide<<acetaldehyde=methanol<mixed solution of methyl iodide and methanol
and that the optimum point of solubility is present in the mixed solution
of methyl iodide and methanol. It has been confirmed that in the
composition of bottom products from the distillation column in the
continuous production process for acetic acid, the recrystallization
temperature is 18.degree. C. at a methyl iodide/methanol weight ratio of
3/1, 12.degree. C. at 5/4, 6.degree. C. at 3/4, and -9.degree. C. or lower
at 1/2. The preferred methyl iodide/methanol weight ratio, which depends
on a thermal insulation state, is 5/4 to 1/2.
A charging position for alcohol can be a charging stage which can be
separated so that alcohol is not lost from the top. It is preferably a
lower part than a charging stage for a mixed solution of acetaldehyde and
methyl iodide, which is subjected to the processing of the present
invention.
The generation and decomposition of paraldehyde and metaldehyde seem to be
influenced by the strength of coexisting acids as well as temperature and
time.
In the present invention, the amount of acetaldehyde to be removed is an
amount by which an acetaldehyde concentration in a reaction liquid during
a steady continuous reaction can be maintained at 400 ppm or less
(preferably 350 ppm or less, more preferably 300 ppm or less).
Essentially, it is the whole amount of acetaldehyde generated under steady
continuous reaction conditions, that is, an amount almost equivalent to an
acetaldehyde conversion amount which is the same as the total amount of
propionic acid, crotonaldehyde, 2-ethylcrotonaldehyde, and hexyl iodide
which are generated in a steady continuous reaction state. Actually,
because propionic acid is the most in terms of quantity and accounts for
the majority, the acetaldehyde amount almost corresponding to a molar
amount of propionic acid can be withdrawn. That is, acetaldehyde can be
withdrawn from a process liquid not only to reduce organic iodides and
carbonyl impurities originating in acetaldehyde contained in product
acetic acid but also to reduce the propionic acid content as well, which
provides the advantage that acetic acid is easily refined.
According to the method in the present invention described above, a trace
impurity concentration in a product can be reduced by reducing an
acetaldehyde concentration in a reaction liquid.
However, when stricter quality is required, an acetaldehyde concentration
in a reaction liquid has to markedly be reduced, which in turn requires
the expansion of facilities such as a distillation column, an extraction
column and a reactor. Accordingly, a large amount of investment in plants
and equipment is required.
Thus, in the case described above, it is preferred that the method of the
present invention and, for example, the method of treating acetic acid
obtained by Macro-reticulated strong-acid cation exchange resin partially
converted to the silver form (U.S. Pat. No. 4615806) are both used.
That is, liquid acetic acid obtained by maintaining the acetaldehyde
concentration in the carbonylation reaction liquid at 400 ppm or less are
contacted with strong acid cation exchange resins in which at least 1% of
the active sites is substituted with silver and/or mercury forms. The
liquid to be contacted with the strong acid cation exchange resins may be
any liquid as long as it contains acetic acid as principal components. A
process liquid having as low a methyl iodide concentration as possible is
preferably used in order to protect the resins. In the present invention,
acetic acid obtained via the line 17 passing through known processes such
as distillation, are contacted with the specific strong acid cation
exchange resins described above with obtain high purity acetic acid
without employing any after-steps such as distillation. Or, they may be
contacted to the specific strong acid cation exchange resins before
passing through known processes such as distillation. Acetic acid coming
out through the line 17 may be subjected to an operation such as
distillation according to necessity after contact with to the strong acid
cation exchange resins.
The strong acid cation exchange resins described above used for removing
impurities such as iodides and the like are of a non-gel type and have
active sites at least 1% of which is substituted with silver and/or
mercury forms. An ion exchange membrane having active sites, at least 1%
of which is substituted with silver and/or mercury forms, an ion exchange
fiber, and polymer resins having functional groups forming coordinate
complexes with silver and mercury, such as a polyvinylpyridine resin, can
be used as well in place of the ion exchange resins described above.
With respect to the amounts of silver and/or mercury bonded to the resins,
at least 1% of the active sites can be converted to the silver and/or
mercury forms, and about 1% to 100% of the active sites can be converted
to the silver and/or mercury Forms. About 25 to about 75% can preferably
be converted to the silver and/or mercury forms.
The temperatures at which strong acid cation exchange resins like this are
contacted with acetic acid are not specifically limited, and they can be
contacted at every temperature extending widely from almost freezing
points of liquid acetic acid to the decomposition temperatures of the
resins. The temperature in practical use is usually about 17.degree. to
about 100.degree. C., preferably 17.degree. to 80.degree. C.
Further, in addition to contacting with strong acid cation exchange resins,
liquid acetic acid obtained by maintaining the acetaldehyde concentration
in the carbonylation reaction liquid at 400 ppm or oxidation treatment
such as ozonation, alkaline metal salt treatment or silver compound
processing may be applied according to necessity. When these treatments
are carried out, though the order of the respective treatment is not
critical, the oxidation treatment is preferably carried out after the
treatment of contacting with the cation exchange resins in order to
prevent the cation exchange resins from being irreversibly swollen.
Higher quality acetic acid, which previously has been uneconomical by any
possibility to be achieved through the removal of acetaldehyde, can be
achieved by combining the removal of acetaldehyde from the system with
treatment by contact with the specific cation exchange resins. In
addition, the amount of the ion exchange resins required for obtaining
acetic acid having sufficiently satisfactory quality can be minimized. In
other words, the ion exchange resin amounts which have so far been used
can be used longer in the present process than in the past.
EXAMPLE
Examples will be shown below to specifically explain the process of the
present invention, but the present invention will not be limited by these
examples. Parts shown in the examples mean weight parts unless otherwise
described.
In the following examples, while test equipment for producing acetic acid
shown in FIG. 1 was operated with a reaction liquid having the
composition: methyl iodide of 14 weight %, water of 8 weight %, methyl
acetate of 1.6 weight %, acetic acid of 70.9 weight %, lithium iodide of 5
weight %, and rhodium of 400 ppm, a part of the lower phase liquid 30 in
the separator bath obtained after condensing the overhead 20 withdrawn
from the methyl iodide-acetic acid splitter column was distilled in a
distillation column of 80 plates in the following conditions to obtain an
acetaldehyde concentrate from the top, and carbonyl impurities were
removed from this concentrate.
______________________________________
Composition of charged liquid:
Methyl iodide 89.4 weight %
Methyl acetate 5.0 weight %
Acetic acid 5.0 weight %
Water 0.5 weight %
Acetaldehyde 0.07 weight %
Paraldehyde 0 weight %
Alkanes 0.01 weight %
Others 0.02 weight %
Distillation condition:
Reflux ratio 170
Charged amount 100 parts (285 kg/hr)
Withdrawn amount 0.19 part from top, 99.81
parts from bottom
Charging plate 70th plate from top
Top temperature 54.degree. C.
Bottom temperature 82.degree. C.
Top withdrawn liquid composition:
Methyl iodide 68.3 weight %
Methyl acetate 0 weight %
Acetic acid 0 weight %
Water 0.7 weight %
Acetaldehyde 29.0 weight %
Paraldehyde 0.1 weight %
Alkanes 1 weight %
Others 0.9 weight %
______________________________________
Removal of this top withdrawn liquid from the system makes it possible to
control the acetaldehyde concentration in the reactor but because the
methyl iodide concentration is high, there is a problem of the loss
thereof or an environmental problem caused by the disposal thereof, and
usually it is not preferable. Accordingly, a water extraction operation
was carried out as shown in the following examples to obtain high purity
acetic acid.
Example 1
It will be shown in the present example that the top withdrawn liquid from
the 80 plate distillation column described above is used to carry out
water extraction and that the extract thus obtained can be distilled to
separate acetaldehyde. Extraction was carried out with a ratio S/F of
water which was a solvent to the top withdrawn liquid from the 80 plates
distillation column described above being set to 1 (weight ratio) and a
theoretical plate of two plates. The extractability of acetaldehyde was
98%. Acetaldehyde of 154 g/hr could be removed by processing the whole
amount of 540 g/hr of the top withdrawn liquid from the 80 plates
distillation column described above. This could lead to removal of 57% of
the amount of 270 g/hr of forming acetaldehyde in the reactor. A raffinate
(methyl iodide-rich liquid) which had been refined by removing
acetaldehyde was recirculated into the tenth plate from the top of the
above 80 plate distillation column to thereby recirculate it into the
reactor as a bottom withdrawn liquid from the above 80 plates distillation
column. An extract (aqueous phase stream) with which acetaldehyde had been
extracted was supplied to the subsequent distillation column, wherein
acetaldehyde was withdrawn as a distillate, and water was withdrawn as a
bottom product. In this distillation, separation could sufficiently be
made at a theoretical plate of 8 plates and a reflux ratio of 0.3. With
respect to operating pressure, any pressure can be used, and the operating
pressure is not essential in this process. Water withdrawn from the bottom
was recirculated to the extractor as a solvent. The acetaldehyde
concentration in the reactor was 200 ppm. As a result thereof, the
permanganate time of product acetic acid obtained was 200 minutes. A wet
product stream withdrawn from the vicinity of the bottom of the methyl
iodide--acetic acid splitter column 14 was dried by distillation. The
concentrations of hexyl iodide and propionic acid in this dried product
liquid were 9 ppb and 270 ppm, respectively.
There are shown extraction materials (top withdrawn liquid), extracts,
raffinates, distillates, and bottom products in Table 1, the compositions
of the lower phase liquid 30 in the separator and the composition of the
recirculating liquid to the reactor in Table 2, and the composition of the
reaction liquid in Table 3, respectively.
TABLE 1
__________________________________________________________________________
Composition (weight %)
Extraction material
Extract
Raffinate
Distillate
Bottom liquid
__________________________________________________________________________
Methyl iodide
68.3 1.0 97.0 4.2 0
Formic acid
0 0 0 0 0.2
Water 0.7 76.8 0.2 2.4 99.8
Acetaldehyde
29.0 21.8 0.8 91.4 0
Paraldehyde
0.1 0 0.1 0 0
Alkanes
1.0 0 1.5 0 0
Others 0.9 0.5 0.4 2.0 0
__________________________________________________________________________
TABLE 2
______________________________________
Composition (weight %)
Lower phase liquid in
Recirculating
liquid separator
liquid to reactor
______________________________________
Methyl iodide
89.4 89.4
Methyl acetate
5.0 5.0
Acetic acid
5.0 5.0
Water 0.5 0.5
Acetaldehyde
0.07 0.016
Paraldehyde
0 0
Alkanes 0.01 0.01
Others 0.02 0.0
______________________________________
TABLE 3
______________________________________
Reaction liquid composition
______________________________________
Acetaldehyde 200 ppm
Methyl iodide 14 weight %
Water 8 weight %
Methyl acetate
1.6 weight %
Hydrogen iodide
0.5 weight %
Acetic acid 70.9 weight %
Lithium iodide
5 weight %
Rhodium 400 ppm
______________________________________
Example 2
A water-extracted processing amount of the acetaldehyde concentrate
obtained from the lower phase liquid 30 in the liquid separator was
changed in the same manner as that in Example 1 to change the acetaldehyde
amount which was removed from the system as shown in Table 4. A
non-processed acetaldehyde concentrate was recirculated into the reactor
as a process liquid. This allowed the acetaldehyde concentration in the
reaction liquid to be controlled as shown in Table 4 without changing the
main composition in the reaction liquid. The concentrations of trace
impurities contained in the dehydrated product acetic acid versus the
acetaldehyde concentrations in the reaction liquid, and the permanganate
time of product acetic acid obtained by further distilling dehydrated
product acetic acid for removing high boiling matters are shown in Table
4.
TABLE 4
__________________________________________________________________________
Reaction
Dehydrated
Removed
liquid AD
product acetic acid Permanganate time of
AD amount
concentration
HexI
CR ECR BA PA product acetic acid
(g/hr)
(ppm) (ppb)
(ppm)
(ppm)
(ppm)
(ppm)
(minute) Remark
__________________________________________________________________________
13 800 100 4 5 17 620 40 Comparative
50 500 50 2.5 1.6 8.5 520 85 Example
131 300 13 1.4 0.4 3.7 330 140 Invention
140 250 11 1.1 0.3 3.1 310 180 Example
154 200 9 0.8 0.1 2.0 270 200
__________________________________________________________________________
AD--Acetaldehyde
HeXI--Hexyl iodide
CR--Crotonaldehyde
ECR--2-Ethylcrotonaldehyde
BA--Butyl acetate
PA--Propionic acid
As shown in Table 4, it can be found that the concentrations of
crotonaldehyde, 2-ethylcrotonaidehyde, butyl acetate, and propionic acid,
as well as hexyl iodide, are rapldly reduced and the permanganate time is
increased to a large extent by setting the acetaldehyde concentration in
the reaction liquid to 400 ppm or less.
Example 3
It will be shown in the present example that acetaldehyde can be extracted
with water at theoretical stages of one stage and two stages, even if the
acetaldehyde concentrations are low.
A liquid obtained by diluting the top withdrawn liquid from the 80 stage
distillation column with methyl iodide was used. Extraction, which was
carried out at an S/F weight ratio of 0.5 and a theoretical stage of one
stage, resulted in obtaining an acetaldehyde extractability of 68%.
Extraction, which was carried out at a theoretical stage of two stages,
resulted in obtaining an acetaldehyde extractability of 95%.
These results are shown in Table 5.
TABLE 5
__________________________________________________________________________
Composition of
Composition at theoretical
Composition at theoretical
extraction material
stage of one stage (weight %)
stage of two stages (weight %)
(weight %)
Extract Raffinate
Extract Raffinate
__________________________________________________________________________
Methyl iodide
85.5 0.9 91.9 1.1 96.3
Water 0.2 85.5 0.1 73.6 0.2
Acetaldehyde
9.3 10.2 2.9 21.3 1.2
Others 5.0 3.4 5.1 4.0 2.3
__________________________________________________________________________
Example 4
While operating the test equipment for producing acetic acid shown in FIG.
1 and 2, the lower phase liquid 30 in the liquid separator obtained after
condensing the overhead 20 from the methyl iodide-acetic acid splitter
column 14 was introduced into the seventieth stage from the top of the
distillation column 40 having the total of 80 stages (Sieve Tray) and
distilled under the conditions of a reflux ratio of 270, a top temperature
of 54.degree. C., and a bottom temperature of 82.degree. C. Setting a
charged amount to 100 parts, 0.33 part was withdrawn from the top, and
99.67 parts from the bottom. For reasons of facilities, a top distillate
from the distillation column 40 was charged into the second distillation
column 60 and distilled under a condition which did not generate
paraldehyde and metaldehyde, that is, a top temperature of 56.degree. C.,
a reflux tank temperature of 32.degree. C., and a top pressure of 2.5
kg/cm.sup.2 G. The distillation column 60 was a packed column having a
theoretical stage of 8 stages, and the total amount of the top distillate
from the distillation column 40 was charged into the fourth stage from the
top. The reflux ratio was 40, and the bottom temperature was 74.degree. C.
Regulating a charged amount to the distillation column of 60 to 100 parts,
38.5 parts of an acetaldehyde concentrate (acetaldehyde concentration:
88.1 wt %) was separated and removed from the top, and 61.5 parts of a
liquid rich in methyl iodide (methyl iodide: 82.8 wt %) were withdrawn
from the bottom as a bottom product, which was recirculated into the
distillation column 40. The concentration of hexyl iodide contained in the
product acetic acid was 28 ppb.
The composition of the reaction liquid is shown in Table 6, the
compositions of the charged liquid to the distillation column 40 and the
top withdrawn liquid from the distillation column 40 are shown in Table 7,
and the compositions of the charged liquid to the distillation column 60,
the distillate from the distillatlon column 60, and the bottom products
are shown in Table 8, respectively.
TABLE 6
______________________________________
Reaction liquid
Composition (wt %)
______________________________________
Acetic acid 70.9
Methyl iodide 14.0
Water 8.0
Methyl acetate
1.6
Rhodium 400 ppm*
Lithium iodide
5.0
Acetaldehyde 345 ppm*
______________________________________
*Unit: ppm
TABLE 7
______________________________________
Composition (wt %)
Charged liquid
Top withdrawn liquid*
______________________________________
Methyl iodide
89.2 53.4
Methyl acetate
5.0 0.1
Acetic acid
5.0 0
Water 0.5 0.5
Acetaldehyde
0.12 40.1
Paraldehyde
0 0.6
Metaldehyde
0 0
Alkanes 0.01 0.2
Others 0.2 5.1
______________________________________
*Distillate
TABLE 8
______________________________________
Composition (wt %)
Charged liquid
Distillate
Bottom liquid
______________________________________
Methyl iodide
53.4 6.3 82.8
Methyl acetate
0.1 0 0.2
Water 0.5 0 0.8
Acetaldehyde
40.1 88.1 11.1
Paraldehyde
0.6 0 0
Metaldehyde
0 0 0
Alkanes 0.2 0.5 0.02
Others 5.1 5.1 5.1
______________________________________
Comparative Example 2
The overhead 20 withdrawn from the methyl iodide--acetic acid splitter
column was circulated into the reactor as it was without distilling. As a
result thereof, the concentration of acetaldehyde in the reactor was 800
ppm, acetaldehyde was not separated and removed, and the concentration of
hexyl iodide contained in product acetic acid was 100 ppb. Also, a wet
product stream withdrawn from the vicinity of the bottom of the methyl
iodide-acetic acid splitter column was dried by distillation, and the
concentration of propionic acid contained in this dried product liquid was
620 ppm.
Example 5
The top withdrawn liquid from the distillation column 40 in Example 1 was
introduced into the distillation column 60 and distilled under conditions
which generate paraldehyde and metaldehyde, that is, a. top temperature of
28.7.degree. C. and a reflux tank temperature of -10.degree. C. The other
conditions in the distillation column 60 were a reflux ratio of 15, a
bottom temperature of 64.6.degree. C. and a top pressure of 1.033
kg/cm.sup.2 in an oldershaw having 20 total stages. Further, methanol of
100 parts was introduced into the seventeenth stage from the top of the
distillation column 60. The top liquid of 100 parts withdrawn from the
distillation column 40 was introduced into the distillation column 60, and
74 parts was withdrawn from the top, which was recirculated into the top
of the distillation column 40. The remaining 26 parts was separated and
removed as a bottom liquid. Methanol was charged to prevent a nozzle at
the lower part of the distillation column from being clogged and enable
the bottom liquid to be withdrawn.
The concentration of hexyl iodide contained the product acetic acid was 40
ppb.
Further, the concentration of acetaldehyde in the reactor was 400 ppm.
The composition of the charged liquid to the distillation column 60, the
distillate from the distillation column 60 and the bottom liquid are shown
in Table 9.
TABLE 9
______________________________________
Composition (wt %)
Charged liquid
From 67
From 44 Distillate
Bottom liquid
______________________________________
Methyl iodide 53.4 69.1 1.8
Methyl acetate 0.1 0.06 0.06
Water 0.5 0 0.4
Acetaldehyde 40.1 22.4 5.1
Paraldehyde 0.6 0.02 0.7
Metaldehyde 0 0 13.4
Alkane 0.2 0.2 0
Others 5.1 6.9 0
Methanol 100 1.5 78.5
______________________________________
Comparative Example 3
The same procedure as that in Example 5 was repeated, except that a
methanol solution was not charged. The result was that the deposition of
metaldehyde crystal clogged the nozzle at the lower part of the
distillation column and prevented operation.
Example 6
Conventional separating operations, that is, dehydrating and distillation
operation for removing high boiling matters were applied to the crude
acetic acid liquid obtained in Example 1 (obtained from the distillation
column in FIG. 1 via the line 17) to find that crotonaldehyde of 0.8 ppm,
2-ethylcrotonaidehyde of 0.1 ppm and hexyl iodide of 9 ppb were contained
in the acetic acid.
This crude acetic acid was passed through the ion exchange resin column at
the temperature of 30.degree. to 40.degree. C. The ion exchange resins
used here were obtained by exchanging 50% of the active sites of RCP 160M
(macroporous strong acid cation exchange resin, manufactured by Mitsubishi
Chemicals Co., Ltd.) with silver. Crotonaldehyde of 0.8 ppm and
2-ethylcrotonaidehyde of 0.1 ppm were contained in product acetic acid
obtained after ion exchange processing. Hexyl iodide was contained at 4
ppb or less. A resin amount required for maintaining this product quality
for one year was an amount corresponding to SV=60 (Hr.sup.-1) in terms of
SV.
Example 7
Crude acetic acid obtained in the above Example 6 after removing high
boiling matters was passed through the ion exchange resin column at the
temperature of 30.degree. to 40.degree. C. The ion exchange resins used
here were obtained by exchanging 42% of the active sites of Amberlist 15
(macroporous strong acid cation exchange resin, manufactured by Rohm &
Haas Co., Ltd.) with silver. Crotonaldehyde of 0.8 ppm and
2-ethylcrotonaldehyde of 0.1 ppm were contained in the product acetic acid
obtained after ion exchange processing. Hexyl iodide was contained at 4
ppb or less. A resin amount required for maintaining this product quality
for one year was an amount corresponding to SV=80 (Hr.sup.-1) in terms of
SV.
Example 8
Crude acetic acid obtained in the above Example 6 after removing high
boiling matters was passed through the ion exchange resin column at a
temperature of 70.degree. to 80.degree. C. and a SV=60 (Hr.sup.-1). The
ion exchange resins used here were obtained by exchanging 50% of the
active sites of RCP 160M (macroporous strong acia cation exchange resin,
manufactured by Mitsubishi Chemicals Co., Ltd.) with silver.
Crotonaldehyde of 0.8 ppm and 2-ethylcrotonaldehyde of 0.1 ppm were
contained in product acetic acid obtained after ion exchange processing.
Hexyl iodide was contained in 4 ppb or less. The ion exchange processing
could be continued in this condition while maintaining the product quality
described above for 2 years.
Example 9
Crude acetic acid obtained in the above Example 6 after removing high
boiling maters was passed through the ion exchange resin column at a
temperature of 70.degree. to 80.degree. C. and a SV=80 (Hr.sup.-1). The
ion exchange resins used here were obtained by exchanging 42% of the
active sites of Amberlist 15 (macroporous strong acid cation exchange
resin, manufactured by Rohm & Haas Co., Ltd.) with silver. Crotonaldehyde
of 0.8 ppm and 2-ethylcrotonaldehyde of 0.1 ppm were contained in product
acetic acid obtained after ion exchange processing. Hexyl iodide was
contained at 4 ppb or less. The ion exchange processing could be continued
in this condition while maintaining the product quality described above
for 2 years.
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